Method of detecting pilot tones in a noisy signal

ABSTRACT

This invention relates to detecting a pilot tone in an optical fiber. In the method described in the invention, computational complexity is reduced significantly by almost completely removing the need to carry out multipli-cations. To calculate certain spectral components of DFT (Discrete Fourier Transform) only a few multiplications are required. The idea is that for de-tecting one pilot tone, it is adequate to calculate one spectral component of DFT if a specific ratio can be chosen between the pilot tone frequency and the sample rate of the receiver.

FIELD OF THE INVENTION

[0001] This invention relates to detecting and measuring a pilot tone inan optical fiber. The optical fiber may contain many optical channels,each carrying a channel-specific pilot tone.

BACKGROUND OF THE INVENTION

[0002] In optical telecommunication transport networks different opticalwavelength channels can be multiplexed into a single fiber. FIG. 1 showsa transmitting node (1) with a multiplexer (MUX) and transmitters (3)for inserting desired data and pilot signals into the fibers. The figurealso shows a receiving node (2) with a demultiplexer (DEMUX) andreceivers (4). Optical wavelength channels carrying data can be droppedor added in the nodes of the network, i.e. the nodes comprisetransmitting and receiving elements. Fe monitoring which channels arepresent at a given place in the network, pilot tones specific to eachwavelength channel can be superimposed (3) on the wavelength carriers.The carriers can be detected by tapping (5) a portion of the light,typically 10 percent, in a given place into a photodetector. To be moreprecise, the optical tap (5) diverts a small part of the optical powerfor extracting desired pilot tones (or a tone). The optical tap (ortaps) can be located before (5) the demultiplexing (2) of the channels,after the demultiplexing, but before the receivers, or in the receivers.

[0003]FIG. 2 shows an example of an arrangement for detecting a pilottone in the fiber (6). The pilot tone can be detected by filtering thesignal obtained from a photodetector (13) through an optional bandpassfilter (8) tuned to let through the frequency range of the pilot tonesand to block most of the telecommunications data signals. It is alsopossible to tune the filter for letting only the desired pilot tonethrough. Usually, the signal from the photodetector is amplified (7)before the bandpass filter. If the pilot tone has a known modulationdepth, the power of the optical carrier can be measured by measuring theamplitude of the pilot tone. Often, the measuring device (11) is a pieceof digital equipment, such as a digital signal processor. Due to thisthe signal from the bandpass filter must be converted to a digitalformat. The converter (9) used gets a sampling rate from an adjustableoscillator (10). The management system (12) can use the measuredamplitude of the pilot tone for any purposes required. Using pilot tonefacilitates the detection of any purposes required. Using pilot tonefacilitates the detection of the presence or absence of wavelengthcarriers, because neither optical filters nor the examining of thetelecommunication data signal are required if pilot tones are used.

[0004] The pilot tone amplitude has to be low so as not to disturb thedata signal. In addition, the data signal itself requires a broadfrequency spectrum, starting from a few tens of kilohertz up to a fewgigahertz. Furthermore, there can be tens or hundreds of wavelengthchannels of different power levels. Detecting weak pilot tones in anaggregate signal incorporating a multitude of other pilot tones, as wellas noise due to the numerous data channels, is a challenge to thedetection system. Since multiple fibers are connected to each node, thedetection system should be as simple as possible.

[0005] A very sensitive method for detecting a pilot tone is to use aphase-locked loop in a receiver containing a local oscillator that istuned to the pilot tone frequency. The signal from the photodetectormultiplied by the local oscillator yields, after low-pass filtering, theamplitude of the pilot signal. By using the product signal as feedbackinto the local oscillator, the phase of the local oscillator can lockonto the pilot tone phase. In this solution, detecting tens of channelsmay be problematic because it takes time for the receiver to lock ontothe phase of the received signal.

[0006] Another method is to sample and digitize the signal. The FastFourier Transform (FFT) technique is then applied on the data to extractthe pilot tone amplitudes. Normally, calculating fast Fourier transformrequires quite great processor power because a significant number ofmultiplications have to be carried out. For example, for a data set of4096 samples, 49000 multiplications (with complex numbers) have to becarried out. If a powerful processor were not needed, cost savings wouldbe obtained.

[0007] The objective of the invention is to avoid these drawbacks byminimizing the number of calculations while maintaining high sensitivityfor detecting a pilot tone. This is achieved in a way described in theclaims.

SUMMARY OF THE INVENTION

[0008] In the method described in the invention, computationalcomplexity is reduced significantly by almost completely removing theneed to carry out multiplications. To calculate certain spectralcomponents of DFT (Discrete Fourier Transform) only a fewmultiplications are required. The idea is that for detecting one pilottone, it is adequate to calculate one spectral component of DFT if aspecific ratio can be chosen between the pilot tone frequency and thesample rate of the receiver. The clocks of the pilot tone transmitterand receiver have to be nearly synchronized but may have an arbitraryphase difference. Adequate synchronism is achieved simply through theinherent accuracy of, for example, free running crystal oscillators. Thefrequency of the calculated spectral component of DFT is preferably aquarter of the sample rate. Using these values, the correlation betweenthe sampled signal values and the DFT spectral component includes onlyadd or subtract operations with complex numbers. The period forrecording the sample values should be short enough to avoid theaccumulation of the phase difference between the measured pilot tone anda signal whose frequency corresponds to one fourth of the sample rate.As a result the DFT spectral component gives the amplitude of the pilottone, which is compared to specific values for making a decision onwhether the tone is present or not. It is possible to detect differentpilot tones by changing the spectral component and also the sample rate.The management system can quickly get the pilot tone informationconcerning a certain channel. At least a few tens of pilot tones can bemeasured in a second.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the following the invention is described in more detail bymeans of FIGS. 1-6 in the attached drawings where

[0010]FIG. 1 illustrates an example of optical multiplexing anddemultiplexing devices, and inserting pilot tones to optical channelsand tapping fibers for detecting the pilot tones,

[0011]FIG. 2 illustrates an example of an arrangement to measure a pilottone,

[0012]FIG. 3 illustrates an example of how samples associate with thepilot signal to be detected,

[0013]FIG. 4 illustrates another example of how samples associate withthe pilot signal to be detected,

[0014]FIG. 5 shows an instance of the invention in a flowchart form,

[0015]FIG. 6 shows another instance of the invention in a flowchartform.

DETAILED DESCRIPTION OF THE INV NTION

[0016] Normally when solving what spectral components a signal contains,Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) iscalculated. DFT and FFT output in the frequency domain all spectralcomponents that the sample rate and number of samples enable. However,only one spectral component is solved according to the invention. Infact, the component solved by the inventive method is one component ofDFT. By using this component multiplications can be removed. Certainprerequisites must be satisfied for finding the component.

[0017] Let's examine FIG. 3. The sine signal (31) describes a pilot tonethat is to be detected and measured. Assuming that the signal in thefiber contains only the pilot tone (no data) helps to construe FIG. 3.X⁺ _(i), I⁻ _(i), X⁻ _(i), I⁺ _(i), etc. illustrate samples at differentmoments when the value of the signal is recorded and forwarded forcalculating the spectral component. As can bee noticed, the sample rateis four times the frequency of the pilot tone. The sample rate can beanother, but then certain multiplications are needed, as describedlater. Assuming that the amplitude (A) of the tone is 1, the next samplevalues are recorded: X⁺ _(i)=0, I⁻ _(i)=1, X⁻ _(i)=0, and I⁺ _(i)=−1.Putting these values into Equation 1 the amplitude A is 1 over themeasurement period (in this case one signal period) as it should be. Itis worth noting that the equation is reduced from a single frequencycomponent of a discrete Fourier transform where the frequency is ¼ ofthe sample rate. The difference of the X_(i) ⁺ and X_(i) ⁻ valuesrepresents the real amplitude of the signal at this frequency, and thedifference of the I_(i) ⁺ and I_(i) ⁻ values represents the imaginaryamplitude of the signal. The absolute value of the complex number Adescribes the total amplitude of the measured signal at this frequency.Parameter n is the number of pilot signal periods over the measurementperiod. $\begin{matrix}{A = {{\frac{1}{2n}{\sum\limits_{i = 0}^{n - 1}X_{i}^{+}}}\quad - X_{i}^{-} + {j\left( {I_{i}^{+} - I_{i}^{-}} \right)}}} & (1)\end{matrix}$

[0018] The phase of the signal to be measured may be arbitrary. FIG. 4describes the signal with a different phase as compared to the situationin FIG. 3. Now, when the amplitude of the tone is still 1, the samplevalues are: X⁺ _(i)=0,7071, I⁻ _(i)=0,7071, X⁻ _(i)=−0,7071 and I⁺_(i)=−0,7071. Putting these values into Equation 1 the amplitude getsthe value 1 when the absolute value of the complex amplitude iscalculated. It can be shown that the phase of the signal can be anyphase.

[0019] So according to the above mentioned matters, multiplications arenot needed. The prerequisites for this are: knowing the frequency of thedesired pilot tone, selecting a suitable sample rate, andquasi-synchronizing the pilot tone with the sample rate. The frequencyof the pilot tone must be known beforehand for adjusting the bandpassfilter, setting the right sample rate, and quasi-synchronizing the pilottone and sample rate.

[0020] The sample rate should preferably be four times the frequency ofthe pilot tone. Using this rate multiplications are not needed. Thenumber of samples can be other than four per period of the pilot tone.(i.e the sample cycle is the period of the pilot tone in this case.) Forexample 3, 5, 6, etc. samples per period are suitable. However, ifanother rate than four samples per period is used, the equation isaltered since certain coefficients are required for each sample value.This indicates a need for a multiplication engine. However, the samecoefficients are repeated in every period of the pilot tone, so addingand subtraction are sufficient until the end of the recording period,when the accumulated sums are multiplied by the coefficients.

[0021] There can also be p/q samples per period of the pilot tone wherep and q are positive integer values and p/q>2 (i.e. p samples areobtained for every q periods of the measured signal). In this case thenumber of recorded periods of the pilot tone must be a multiple of q. Inthe worst case p sums have to be accumulated and multiplied bynon-integer coefficients at the end of the recording period.

[0022] The sample rate can be smaller than presented above by usingsimilar methods as in sampling oscilloscopes to assign the samples tocertain phases of the signal.

[0023] The frequency of the pilot tone and the sample rate must bequasi-synchronized. The quasi-synchronism is achieved by acquiring alimited number of pilot tone cycles per measurement, for example 1000.During the measurement time the phase of the sample cycle (samplesduring a period of the pilot tone) and the phase of the pilot toneremains approximately the same. This is possible with the accuracy of astandard crystal oscillator, which is typically 50 parts per million.Thus, phase locking is not required. An arbitrary phase differencebetween the transmitter and receiver is accepted because 4 samples perperiod are acquired and samples having a 90 degree phase difference arehandled separately.

[0024] Since the phase difference, as mentioned above, remainsapproximately the same, the samples can be summed directly and nomultiplication as in FFT is required. The calculation of the complexamplitude (A) of the pilot tone is portrayed in equation 1. Theamplitude of the pilot tone is obtained by calculating the absolutevalue of the complex amplitude (A).

[0025] Analog integration of the input voltage can be used whenregistering the samples. The integrating time (FIG. 3, ∫MAX) of eachsample can be very short for obtaining the most accurate value for A ifnoise was not present. The maximum integration time per sample is onefourth of a period (when using four samples per period). At maximumintegration time high-frequency noise is filtered at the expense of aslight error in the measurement. The error can be corrected because itis only about 10% of the signal amplitude and does not depend on thephase difference between the signal and the sample cycle.

[0026]FIG. 5 shows the invention in a flowchart form. First, a suitablesample rate must be selected (51) concerning the pilot tone and theaccuracy needs. The pilot tone frequency (and waveform) is knownbeforehand. The phase of the sample cycle should not drift as comparedto the phase of the pilot tone, so the sample rate is synchronized witha sufficient accuracy to the pilot tone. The accuracy of a crystaloscillator, 50 ppm, typically used as a clock frequency source, isadequate for the measurement, and in such a case no synchronizationprocedure is required. By selecting a suitable (short enough) recordingperiod (52), the accumulation of a phase difference between the pilottone to be measured and the sample rate can be avoided. After thesesteps, the measurement equipment is ready to record (53) the samplevalues, and to accumulate the sums of the values according toequation 1. Depending on the selected sample rate, the coefficients thatare used to multiply the accumulated sums of the sample values in theend must be set (54). The DFT spectral component is calculated (55)concerning the pilot tone for obtaining the amplitude of the pilot tone.It is also possible to think that accumulating the sums (53), settingcoefficients (54), and calculating the spectral component (55) togetherform Fourier transformation for the desired spectral component, butdividing these phases into separate blocks gives a more illustrativepicture. Finally, the calculated amplitude is compared (56) to thepredetermined values which indicated the existence of the pilot tone orthe lack of the pilot tone. The result of this comparison tells whetherthe pilot tone is present or not. It should be noted that leaving thefinal phase (56) out, the method can be used just for measuring theamplitude of the pilot tone.

[0027]FIG. 6 illustrates a preferable embodiment of the invention in aflowchart format. In this case, the sample rate is selected (61) to befour times the frequency of the pilot tone. Using this sample rate, thecoefficients of the sample values do not need to be set (compare phase54 in FIG. 5), and thus multiplying the sums by the coefficients (phase55 in FIG. 5) is not needed. Accumulating the sums of the values (62)corresponds to the spectral component of Fourier transform.

[0028] The arrangement according to the invention is similar to FIG. 2.However, the measuring device (12) does not need to be a powerfulprocessor, but it can be a simpler device. Naturally, this saves costs.

[0029] In the present invention the goal is to minimize thecomputational effort while maintaining high sensitivity. For example, ina case of using FFT (Fast Fourier Transform) for a 4096 sample data set,49000 multiplications (with complex numbers) have to be carried out. InFFT, only a part of the calculations can be carried out before the lastsample is obtained. Thus, the result is obtained after an additionaldelay, after obtaining the last sample.

[0030] On the other hand, in the method according to the invention 4100add or subtract operations with complex numbers are required for thecorrelation procedure to detect one pilot tone from an equivalent dataset of 4096 samples. No multiplications have to be carried out exceptfor normalizing the result. The result of the correlation procedure isavailable almost instantly after the last sample has been obtained.

[0031] However, if a large number of pilot tones are to be detected, thecomputational advantage of the method is lower than in the case of onechannel, because the number of operations increases linearly with thenumber of channels as compared to FFT where all channels are detected bya single transform. On the other hand, the management usually has othermeans to detect problems associated with a certain channel, for example,by detecting the channel dropped at the channel termination.Consequently, the management can instruct all nodes along the trail tomeasure immediately that channel and get the results quickly. Areasonably large number of channels can be monitored continuouslybecause a simple microcontroller is adequate for detecting several tensof channels per second.

[0032] The synchronism between the pilot tone and the sample rateassures that practically the whole signal amplitude is represented bythe spectral component calculated. If the signal is not synchronizedwith the sample rate, a quadratic sum of about 10 spectral components ofDFT have to be calculated to obtain the amplitude of the pilot tone. 10spectral components, approximately doubles the noise-induced error inthe final result. In addition, there will be more cross-talk from otherpilot tones.

[0033] Different pilot tones can be detected by changing the sample rateof the receiver. A very good selectivity between different pilot tonescan be achieved if the pilot tone frequencies are selected so that allpilot tones experience an integer number of oscillations within theduration of the measurement. This can be achieved if the pilot tonefrequencies are equally spaced and the duration of the measurement isthe same for all tones.

[0034] If, however, pilot tone frequencies are obtained by division ofthe frequency of a clock that is identical for all pilot tonegenerators, equal frequency spacing is not achieved. In this case thepilot tones should be arranged in pairs. Frequency spacing between thetones comprising the pair can be smaller than between the neighboringpairs. It is possible to select a measurement period for a pair of pilottone frequencies produced by division of the frequency of identicalclocks, so that both tones experience an integer number of oscillationsduring the period.

[0035] In the proposed system only a few bytes of random access memoryare required, which further reduces the silicon area consumed by thesystem. It is evident that the invention is not restricted to theexamples above, but it can also be used in other solutions, in the scopeof the inventive idea.

1. A method for detecting the presence of a pilot tone, whosecharacteristics are known, in a signal, the method comprising the stepsof sampling the signal, and recording the values of the samples from adesired recording length, characterized in that the method furthercomprises the steps of calculating one spectral component of Fouriertransform for the values of the samples, and comparing the result of thecalculation to at least one predetermined value for deciding whether thepilot tone is present or not.
 2. A method according to claim 1,characterized in that the sampling step includes the step of selecting asuitable sample rate depending on the features of the pilot tone, thesignal, and the sampling step.
 3. A method according to claim 2,characterized in that in the selecting step the sample rate is chosen tobe four times the frequency of the pilot tone.
 4. A method according toclaim 3, characterized in that the calculating step comprises the stepof accumulating sums of the values.
 5. A method according to claim 2,characterized in that when selecting the sample rate to be differentthan four times the frequency of the pilot tone, a step of settingcoefficients for the calculating step is between the step of recordingthe values and the step of calculating one spectral component of Fouriertransform.
 6. A method according to claim 5, characterized in that thecalculating step comprises the steps of accumulating sums of the values,multiplying the sums by the coefficients, and adding multiplied sumstogether.
 7. A method according to claim 1, 2, 3, 4, 5 or 6,characterized by integrating the signal when recording the values forthe samples.
 8. A method according to claim 1, 2, 3, 4, 5, 6 or 7,characterized by reducing crosstalk between pilot tones when severalsignals with pilot tones exist, by equally spacing frequencies of pilottones and selecting a specific measurement time for the pilot tones toensure the reduction of crosstalk.
 9. A method according to claim 8,characterized by selecting an equal measurement time for the pilot tonesto ensure the reduction of crosstalk.
 10. A method according to claim 1,2, 3, 4, 5, 6 or 7, characterized by reducing crosstalk between pilottones when several signals with pilot tones exist, by arranging thepilot tones in pairs with a pair-specific frequency difference and apair-specific measurement time.
 11. A method according to claim 1, 2, 3,4, 5, 6 or 7, characterized by reducing crosstalk between pilot toneswhen several signals with pilot tones exist, by arranging the pilottones in pairs with a specific frequency difference and a specificmeasurement time, both being equal to all pairs.
 12. A method formeasuring a pilot tone, which pilot tone's characteristics are known, ina signal, the method comprising the steps of sampling the signal, andrecording the values of the samples from a desired recording length,characterized in that the method further comprises the steps ofcalculating one spectral component of Fourier transform for the valuesof the samples, and using the calculated spectral component as a measureof the amplitude of the pilot tone.
 13. A method according to claim 12,characterized in that the sampling step includes the step of selecting asuitable sample rate depending on the features of the pilot tone, thesignal, and the sampling step.
 14. A method according to claim 13,characterized in that in the selecting step the sample rate is chosen tobe four times the frequency of the pilot tone.
 15. A method according toclaim 14, characterized in that the calculating step comprises the stepof accumulating sums of the values.
 16. A method according to claim 13,characterized in that when selecting the sample rate to be differentthan four times the frequency of the pilot tone, a step of settingcoefficients for the calculating step is between the step of recordingthe values and the step of calculating one spectral component of Fouriertransform.
 17. A method according to claim 16, characterized in that thecalculating step comprises the steps of accumulating sums of the values,multiplying the sums by the coefficients, and adding multiplied sumstogether.
 18. A method according to claim 12, 13, 14, 15, 16 or 17,characterized by integrating the signal when recording the values forthe samples.
 19. A method according to claim 12, 13, 14, 15, 16, 17 or18, characterized by reducing crosstalk between pilot tones when severalsignals with pilot tones exist, by equally spacing frequencies of pilottones and selecting a specific measurement time for the pilot tones toensure the reduction of crosstalk.
 20. A method according to claim 19,characterized by selecting an equal measurement time for the pilot tonesto ensure the reduction of crosstalk.
 21. A method according to claim12, 13, 14, 15, 16, 17 or 18, characterized by reducing crosstalkbetween pilot tones when several signals with pilot tones exist, byarranging the pilot tones in pairs with a pair-specific frequencydifference and a pair-specific measurement time.
 22. A method accordingto claim 12, 13, 14, 15, 16, 17 or 18, characterized by reducingcrosstalk between pilot tones when several signals with pilot tonesexist, by arranging the pilot tones in pairs with a specific frequencydifference and a specific measurement time, both being equal to allpairs.
 23. An arrangement for detecting the presence of a pilot tone,whose characteristics are known, in a signal, the arrangement comprisingsampling means for sampling the signal, and recording means forrecording the values of the samples, characterized in that thearrangement further comprises means for calculating one spectralcomponent of Fourier transform for the values of the samples, means forcomparing the result of the calculation to at least one predeterminedvalue for deciding whether the pilot tone is present or not.
 24. Anarrangement according to claim 23, characterized in that the arrangementfurther comprises means of setting coefficients of the values of thesamples.
 25. An arrangement according to claim 23 or 24, characterizedin that the recording means further comprises means of selecting arecording length.
 26. An arrangement for measuring a pilot tone, whichpilot tone's characteristics are known, in a signal, the arrangementcomprising sampling means for sampling the signal, and recording meansfor recording the values of the samples, characterized in that thearrangement further comprises first means for calculating one spectralcomponent of Fourier transform for the values of the samples, secondmeans, respective to first means, for recording the value of thecalculated spectral component as a measure of the amplitude of the pilottone.
 27. An arrangement according to claim 26, characterized in thatthe arrangement further comprises means of setting coefficients of thevalues of the samples.
 28. An arrangement according to claim 26 or 27,characterized in that the recording means further comprises means ofselecting a recording length.